The threads used in connecting basing and tubing pipes together in the oil and gas industry is the single most important part of the overall connector design. These threads are required to resist mechanical loads that often can reach over one million pounds but will not seize or gall after repeated make-ups. They determine the overall characteristics of the connector since a problem in the threads can weaken even the best-designed seals and torque shoulders.
Over the last five decades, major advancements have been developed in thread design beyond the original V-shaped thread forms. The most successful threads used in the oil and gas industry have been the tapered V-shaped threads, and these threads were expected to perform all the duties of the connector, including pressure sealing while maintaining mechanical strength. As the industry attempted to drill and complete wells with deeper and deeper depths and higher pressures, the simple V-shaped thread began to show its limits. One of the most popular designs introduced after the V-shaped was the straight, noninterference thread form, which is still very popular today in tubing connectors. This approach removed the sealing duties from the threads by incorporating metal-to-metal interference-fit seals and torque shoulders. By specializing the different parts of the connector for different functions, the connector became a little more predictable. The straight thread has remained popular because of its smooth running characteristics in the field.
The next generation of threads designed into connectors for high-performance pressure sealing and strength was the tapered, buttress-type thread form. These threads have always used radial interference to generate torque that helps resist downhole back-off, which has been one of the weaknesses of the straight thread design. Like the straight threads, these tapered thread bodies were used in connectors that incorporated specialized seals and shoulders.
But even though these two major design categories were successful over numerous decades of use, they each had limits as to their performance capabilities. The straight thread form, with no radial interference, experienced from time to time downhole back-offs caused by heat cycling and rotating the string of pipe. And, while the tapered interference-fit thread did not suffer this weakness, it often generated too much circumferential stress in the box, especially during make-up and from applied internal pressure. Both of these thread forms suffered a weakness that has become a major problem as well pressures continue to rise and the depths continue to go deeper. The problem is that the threads provided no way for the pin and box to lock together radially, which would allow resistance to pressure inside the pipe that somehow could migrate up into the space between the pin and box. These older thread designs also had the tendency to "jump out" under very high tension or bending loads because the amount of metal available on the end of a piece of pipe, even with forged ends or coupling, is never adequate for the incorporation of big, tough-thread teeth that could resist such loads. It is an impossible task to accurately predict the loads these connectors go through 3-5 miles under the surface of the Earth.
These weaknesses have brought on numerous products over the last ten years or so that incorporate a hooked or locking-type thread form. These threads utilize at least one of the load flanks being reverse angled or hooked such that when tension loads are applied, the connectors basically draw together or get tighter. This has been a solid breakthrough for the industry, but these designs all have some weaknesses, mostly with resisting compressive or bending loads. Others that do not have this drawback are so sophisticated that the cost of production is very high.
The weakness in compression or bending with the current products on the market is created by the clearance these forms require on the compression flank of the thread tooth. The hooked tension flanks work great, but when bending (compression on one side and tension on the opposite side) or compression is applied in any great quantities, the thread body has no way of carrying those loads because there is no contact on that side of the thread tooth. This usually means that the torque shoulders have to carry these loads and the shoulders are sometimes pressure seals or are adjacent pressure seals. Axial strain generated from this applied compression can be very detrimental to these seals and shoulders.
This invention includes a thread form design that offers the locking effect so important in today's oil and gas industry but also provides the compressive and bending strength absent in most designs. This means a thread form design that is truly rigid as it can resist extremely high tension loads without experiencing jumpout, is equally capable of carrying the applied compressive and bending loads, and is adequately capable of locking the box and pin together should high gas pressures migrate up into the interstitial space between the pin and box. This invention provides the smooth running and low stress characteristics of the straight or noninterference-fit thread form. This invention is cost effective.